True control over the rate of nutrient release by fertilizers has long been known to be a desirable but elusive goal. A plant's need for nutrients changes from season to season, and species to species. As a result farmers, foresters and gardeners have generally relied on the use of repeated, timed doses of fertilizers in an effort to obtain optimal plant growth. A fertilizer product tailored to a plant's specific needs over a relatively long period of time, would eliminate the need for one or more doses.
Such products would be of great value to nurserymen and foresters, the first because current methods of fertilizing are labor intensive, and the latter because such products could greatly increase the rate of seedling survival.
The invention generally relates to products capable of releasing of water soluble agents such as those exhibiting delayed, controlled release and those having a relatively high initial release followed by a relatively low, constant release. A "delayed, controlled release" is defined as a release for which onset is substantially delayed for a predetermined period of time. Upon onset of release, the agent is released at a controlled rate.
This invention is particularly related to the timed release of agents suitable for cultivating horticultural growth, e.g. fertilizer products. By the term "horticultural", it is meant to include not only the science of growing fruits, vegetables, flowers or ornamental plants, but also to include, but not necessarily to limit to, silviculture, i.e. a branch of forestry dealing with the development and care of forests, and the science of cultivating grasses such as home lawns.
Controlled release products have become an important tool in many fields. It is well known that controlling the rate of release of an active agent from its carrier product maximizes the agent's effect on its target and minimizes potentially harmful side effects. In other words, if it is known that the target not only requires the agent's presence at the time the product is applied, but that the agent will also be required over a specific period of time after the initial application, the product can be modified so that agent is released over the period of time mandated. Products employing controlled release have been used in many fields including, but not necessarily limited to, agriculture, health care, personal care and hygiene, and pharmaceuticals.
In the agricultural field, controlled release products are used to deliver fertilizer, herbicidal and pesticidal agents to crops to maximize the agent's effect over the crop's growing season. See U.S. Pat. No. 3,223,518 issued to Hansen Dec. 14, 1965; U.S. Pat. No. 4,019,890 issued to Fujita et al. Apr. 26, 1977; U.S. Pat. No. 4,015,970 issued to Hennart Apr. 5, 1977; and U.S. Pat. No. 4,851,027 issued to Murayama et al. Jul. 25, 1989. For instance, it is well known that crops in the early stages of development are harmed by heavy doses of fertilizer. In the past, a program comprising several light applications over the growing season had been recommended to solve that problem. However, such programs can be difficult to follow because of the increased labor costs and the incidental crop damage caused by equipment moving through the field during applications which occurred late in the program. Furthermore, when water soluble fertilizers are applied to open fields, some of the fertilizer is washed into the local drainage system by rainwater runoff. This portion of the fertilizer is ineffective for its intended use and can significantly pollute nearby waterways and reservoirs.
As a result, controlled release products which delivered fertilizer at rates which attempt to approximate a program of several light applications have been developed. To a large extent, these products are made by coating fertilizer granules or prills with various materials to reduce the rate of release of the fertilizing agent.
For instance, U.S. Pat. No. 3,223,518 issued to Hansen Dec. 14, 1965 discloses coatings of polymer resins exemplified by linseed oil- or soybean oil-based resins, e.g. linseed oil-based copolymers with dicyclopentadiene. The release rates of the coated products described in the '518 patent depend on various factors, some of which include the number of coatings applied to the product, or the coating's thicknesses, and the type of polymer used in the coating. The controlled release rates obtained from those coated products range from 55% of the fertilizer being released in 24 hours down to about 0.05% in 120 hours. As the graphs which appear in the '518 patent illustrate, the release rates for these coated products can be varied and can be described as a continuous release which begins upon application of the fertilizer product. In other words, the onset of release occurs almost immediately upon application of the fertilizer product and typically within a week of being applied. A fertilizer product exemplifying this type of controlled release is available as Osmocote.RTM. fertilizer from Grace-Sierra Horticultural Products Co.
Another type of coated fertilizer product employs additives to effect controlled release. U.S. Pat. Nos. 4,019,890 and 4,369,055 issued to Fujita Apr. 26, 1977 and Jan. 18, 1983 respectively, describe polyolefin resin coatings that contain hydrophilic powder additives and surfactants which, due to their water-compatibility, create pores within the resin coatings upon contact with soil moisture. However, due to the water-insolubility of the coating resin, e.g. thermoplastics such as polyethylene, polypropylene and copolymers thereof, the coating is not detrimentally affected by the soil moisture, so the coating will not disintegrate, and remains intact. The pores in the coating allow for a low, substantially constant release rate, and similar to the release exhibited by the coated products described in the '518 patent, and the onset of this release occurs upon application of the product. Commercially available fertilizers which employ the additive approach include NUTRICOTE.RTM. fertilizers from Chisso-Asahi Fertilizer Co., Ltd.
U.S. Pat. No. 5,089,041 issued to Thompson et al. Feb. 18, 1992, discloses yet another type of coated fertilizer product which includes a waterborne, polyvinylidene chloride-based latex coating on a water-soluble core. The rate of release is slow and relatively constant, and onset of release begins almost immediately.
Other methods of controlling release employ microcapsules of active ingredients which are soluble in organic solvents. U.S. Pat. No. 4,670,250, issued to Baker Jun. 2, 1987 discloses microcapsules that are prepared from thermoplastic polymers such as polysulfones, polycarbonates and poly(styrene-acrylonitrile)polymers. Again, the active ingredient is delivered at a slow and relatively constant rate.
As illustrated above controlled release products have been developed in several technical fields, especially fertilizers. As also illustrated, the degree of control of release of the fertilizers has mainly been limited to reducing the release rate of active agents, thereby preventing large amounts of fertilizer from being released too early. Such low rates however are also usually continuous for the fertilizer's period of usefulness. In instances where it would be advantageous to increase the rate of fertilizer release after that initial period of slow release, such slower release products do not maximize delivery of fertilizer. In those instances it would be preferable to have a product for which the onset of agent release is delayed for the period of time necessary, but for which at a later, predetermined time substantial release began. Also, a long-term product should be able to provide a period of nearly complete cessation of release during the dormant periods of deep winter and summer.